A. Field of the Invention
The invention relates generally to the field of instrumentation for electrochemical measurement, and in particular to an improved meter for determining pH and concentration values for ionic solutions.
B. Description of the Related Art
Portable analytical instruments such as pH and concentration meters offer significant convenience to laboratory personnel, who must contend with a wide variety of experimental conditions and types of measurement. Typical metering arrangements include an ion-sensitive electrode that the user immerses in an electrolyte solution of interest, support circuitry, and display means for presenting to the user measurements of selected characteristics associated with ionic species in solution.
Currently available instrumentation includes meters capable of measuring hydrogen-ion concentration (generally in pH units) of aqueous solutions, absolute ion concentration (generally in units of equivalents/liter), dissolved oxygen concentration (generally in parts per million), and absolute or relative potential (generally in millivolts, and used in potentiometric titrations). Meters may be configured for visual display only, or adapted for interface to portable printers or even standalone computers.
Despite the versatility of such instrumentation, considerable effort on the part of the manufacturer is often required for assembly, and the units are not always convenient to operate. Conventional analytical electrodes consist of immersible glass tubing assemblies connected by wires to the analytical instrument. The electrode is usually first placed into the sample solution, and the meter subsequently positioned for reading. Repetition of this two-step procedure can cause wires to become dislodged in the course of experimental work, or result in electrode breakage if the metering apparatus is suddenly moved. Users also often misplace electrodes upon disengagement thereof from the apparatus.
Conventional packaging for metering instruments is often more expensive than necessary, because designers tend to neglect component interrelationships that can be exploited to facilitate modular assembly. For example, circuit boards, output ports, power supply and display components are often independently bolted into the main housing at various unrelated points, requiring separate installation operations for each.
A fundamental operational requirement of all electrochemical measurement instruments is user-performed calibration, which readiews the meter for each use. Calibration is generally accomplished by sequentially immersing the electrode in each of a group of "reference solutions" of known pH or ionic concentration values and having temperature levels similar to those of the electrolyte solutions that will be measured. The current flows sensed by the meter as the electrode is introduced into each reference solutions are used as data points to orient a calibration curve that relates current flow to pH or concentration. This curve, based on the well-known Nernst equation, has a known, characteristic shape. With the dimensions and location of the calibration curve established by the reference measurements, values for pH or concentration can be obtained from the observed current flows obtained by immersing the electrode in solutions of interest.
One limitation associated with many known calibration systems is the narrow range of values along the curve that may be used as data points for curve orientation. The narrower the range of reference values, the less reliable will be the resulting calibration curve due to ambiguities in its orientation. A second limitation relates to noise associated with the meter circuitry itself which, if not accounted for during calibration, can jeopardize the validity of measurements.